1. Field of the Invention
The present invention relates to an inner-shield material to be attached inside a color cathode ray tube and its manufacturing method, especially to the manufacturing method for the inner-shield material having superior magnetic characteristics and rust protection or corrosion resistance that eliminates a blackening process in its manufacturing method.
The magnetic shield material for a color cathode ray tube is attached to an outer or inner side of the color cathode ray tube to prevent electron beams from being deflecting by terrestrial magnetism. The magnetic shield material attached inside the tube is referred to as an inner shield material, while that outside the tube is referred to as an outer-shield material. In addition to such magnetic characteristics as higher permeability and lower coercive force, the characteristics on thermal radiation and rust protection are required for the magnetic-shield material.
For this purpose, for example, in Japanese Patent Publication No. 1894 of 1989 (hereafter referred to as "the former reference"), the use of Ni- or Cr-plated steel sheet or strip as inner-shield materials and the technique for blackening the surface of the plated steel sheet or strips in the heat treatment on the color cathode ray tube manufacturing process have been disclosed. However, this technique is only for manufacturing color cathode ray tubes, it could not be extended to manufacturing inner-shield materials. On the other hand, TOKU KAI HEI 2 (Japanese Unexamined Patent Publication 1990) 228466 (hereafter referred to as "the latter reference") disclosed a technique for forming a blackened film or coat with FeO contained as its main constituent on the surface of a thin steel sheet or strip using an oxidizing gas and a non-oxidizing gas in a continuous annealing line in an inner-shield material manufacturing process. The latter reference mentioned that this technique could eliminate the blackening process in the manufacture of color cathode ray tube.
The former reference suggests that the use of Ni- or Cr-plated steel sheet or strips may eliminate the blackening process which has been necessary for conventional non-plated steel sheet or strip. In effect, some manufacturers use ultra-thin Cr-plated steel sheet or strips as an inner-shield material.
Usually, however, the ultra-thin Cr-plated steel sheet or strips described in the former reference are produced through a sequential process of annealing, skin pass rolling, and plating. Thus, ultra-thin Cr-plated steel sheet or strip have the disadvantage of inferior magnetic characteristics inasmuch as the grains of the annealed steel sheet or strip are distorted by the skin pass rolling. It was found that these magnetic characteristics by themselves could be improved by modifying the manufacturing process to use the sequential processes of plating and annealing. However, nobody has used this process because of some difficulties in the plating step following annealing, as discussed below.
That is to say, the material, prior to being subjected to the plating process, has been softened through the annealing process, and moreover, its thickness is very thin (in general, steel sheet or strips with a thickness of 0.15 mm are used).
Thus, the material cannot be passed through the plating process because of the so-called "wrinkle" on its surface. Or even when it may pass through the process, the material is so deformed that it is unusable as an inner-shield material.
On the other hand, the method described in the latter reference is considered to be better than that the method described in the former reference with respect to providing good magnetic characteristics.
The blackening is executed in the continuous annealing furnace in accordance with the following steps.
1 Heating process PA0 2 Soaking process PA0 3 Cooling process
Forming Fe.sub.3 O.sub.4 in an oxidizing gas PA1 Transforming Fe.sub.3 O.sub.4 into FeO in a non-oxidizing gas PA1 Rapidly cooling the steel sheet or strip in the non-oxidizing gas atmosphere to form the blackening film containing FeO as the principal constituent PA1 a) The heating pattern and the gas atmosphere should be strictly controlled to form a blackening film which can withstand to the press forming process. PA1 b) The blackening film containing FeO as a principal constituent should be formed on the surface of the material under strictly controlled conditions.
This method is a new technique for improving the blackening film's adhesion by transforming the Fe.sub.3 O.sub.4 having inferior adhesion to the material into FeO, which can eliminate the blackening process in color cathode ray tube manufacturers.
The latter reference, however, suggests the following:
These suggestions mean that in the method of the latter reference, it is likely that Fe.sub.3 O.sub.4 with inferior adhesion may be occasionally formed.
Thus, in the latter reference, some manufacturing and quality assurance problems exist because Fe.sub.3 O.sub.4 with inferior adhesion might be produced if strict control over manufacturing conditions is not maintained.
The object of the present invention is to provide both an inner-shield material and with superior magnetic characteristics and rust protection and a method for its manufacture which does not require a conventional blackening process.
Another object of the present invention is to provide an inner-shield manufacturing method, by which steel sheet or strip passing is made smooth in the plating process and the Ni-plated steel sheet or strip is prevented from sticking in the annealing process.